Development of New Flame Retardant and Heat Stabilized Polyamides

Abstract

This work is concerned with the synthesis and application of novel polymeric flame retardants for polyamides based on phosphorus functionalities. The flame retardants are primarily formed using the reactive P–H bond to form monomers that then can be polymerized. One promising approach was the Phospha-Michael addition of 9,10-dihydro-9-oxa-10-phospha-phenanthrene-10-oxide (DOPO) with zinc diacrylate in order to combine different flame retardancy mechanisms. A polymer and a low-molecular weight complex were synthesized for comparison. Interestingly, the phosphorus moiety, which usually acts in a gas-phase mechanism in the case of DOPO, acted mainly in the condensed phase for the resulting substances. Both products were incorporated into polyamides in varying concentrations and with different synergists. A drop in melt viscosity during compounding was observed and the effect was investigated. The flame retardant properties of the tested materials were a V-2 rating due to dripping and thus not sufficient except for one sample for which a UL94 V-1 rating was reached. In a second topic, this work deals with the combination of heat stabilization with flame retardancy in a one-molecule approach. Three different tactics were pursued. First, phosphorylated polyols (P-Polyols) were generated. Therefore, different phosphorus-containing diols were synthesized as precursors and then fused with polyols in a solvent-free melt reaction. The produced P-Polyols where incorporated into polyamides as a flame retardant synergist in different concentrations together with diethyl phosphinate aluminum (DEPAL) in order to find a concentration at which some amount of DEPAL can be replaced with the new flame retardant/heat stabilizer combination. Also, the heat stabilization effect in polyamide was tested for one of the P-Polyols in comparison with a phosphorus-free reference polyol. The results showed that the new P-Polyol works as a heat stabilizer but is not as efficient as the reference. Also, it was found that both the reference and the self-synthesized substance only work in glass fiber-reinforced polyamide. However, P-Polyols seem to be a promising approach. Another tactic for the combination of heat stabilizer and flame retardant was the copolymerization of different hydroxyl- and phosphorus-containing acrylates and acrylamides. A variety of homopolymers and copolymers were synthesized and compared with respect to thermal stability and residue. It was expected that a higher amount of residue will lead to better heat stability. To test the feasibility of this approach, two non-phosphorus-containing polyacrylamides with different amounts of hydroxyl units were incorporated into polyamide 6 and were tested for heat stabilization. Both polymers showed insufficient stabilization behavior and thus this approach was not pursued further. As a last approach the Pudovik reaction of DOPO with the bio-based aldehydes acrolein and hydroxymethylfurfural was conducted. This reaction generated alpha-hydroxyphosphinates with which polymerization via radical polymerization and polycondensation was attempted. However, neither polymerization was successful. Different functionalizations were performed on the DOPO-acrolein adduct in order to make it stable enough to incorporate in polyamides as a flame retardant. However, non of these functionizations yielded a sufficiently thermally stable product.